The transmission and spread of most bird-associated arboviruses is poorly understood, in part because of the difficulty in reliably locating both hosts and vectors for sampling. Arthropod-borne alphaviruses are responsible for a variety of zoonotic human illnesses throughout the world, known or suspected to cause encephalitis, polyarthritis, hemorrhagic fever, and a variety of less serious diseases. Yet, what causes epidemics to occur in some areas, and how a virus persists in the interim between epidemics, is not well known. This study investigates the population dynamics of a recombinant alphavirus, Buggy Creek virus BCRV), in a natural system, in which the virus is associated with a vertebrate host, the cliff swallow Petrochelidon pyrrhonota), and an invertebrate vector, the cimicid swallow bug (Oeciacus vicarius). The project will determine the mechanisms that allow annual occurrence of the alphavirus in the population reintroduction by migratory birds vs over wintering in sedentary bugs), the extent of inter-group transmission by both birds and bugs, and how annual infection rates and transmission vary with ecological characteristics of colony sites. The virus subtypes at different sites will be characterized by sequencing, and the extent of evolutionary divergence at different sites will be related to patterns of site use and other ecological characteristics. The potential role of mosquitoes as a bridging vector to humans will be assessed. The work will rely on field collections of bugs for virus isolation and blood collection from birds, and will take advantage of a long-term dataset consisting of over 148,000 marked birds, for which survival and movement within the study area can be estimated. The project is one of the few to be able to investigate alphavirus transmission, persistence, and evolutionary divergence in a natural system where the foci of virus infections (i.e., colony sites) can be reliably identified and ecological hypotheses tested.